Synthesis, characterization, DFT calculations, and electrochemical comparison of novel iron(ii) complexes with thione and selone ligands

Abstract

Thione- and selone-containing compounds and their metal complexes show promise as antioxidants, as antithyroid drugs, and for applications in lasers and blue light-emitting diodes. Although Cu(i/ii), Co(ii), Ag(i), and Zn(ii) coordination to thione and selone ligands has been broadly studied and Fe(ii) plays an important role in oxidative damage, very few iron-thione complexes and no iron-selone complexes are reported. Novel Fe(ii)-containing thione and selone complexes of the formulae FeL2Cl2, [FeL2(CH3CN)2](2+), and [FeL4](2+), and {FeL'Cl2}n, (L = N,N'-dimethylimidazole selone (dmise), and thione (dmit); L' = bis(thioimidazolyl)ethane (ebit) and bis(selenoimidazolyl)ethane (ebis)) have been synthesized and characterized. Structures of Fe(dmise)2Cl2, Fe(dmit)2Cl2, [Fe(dmit)4][BF4]2, [Fe(dmise)4][BF4]2, and {Fe(ebit)Cl2}n were determined by X-ray crystallography. All Fe(ii) centers adopt a distorted tetrahedral coordination geometry with Fe-S distances ranging from 2.339(1) to 2.397(1) Å and F-Se distances ranging from 2.453(1) to 2.514(1) Å. Density functional theory optimized structures of FeL2Cl2, [FeL2(CH3CN)2](2+), and [FeL4](2+) are consistent with experimental results and suggest that thiones and selones are π-donor ligands that coordinate through their zwitterionic resonance structures. Thione and selone coordination to Fe(ii) lowers the Fe(ii/iii) reduction potential, with a greater decrease for Fe(ii)-bound dmise than Fe(ii)-bound dmit. Dmit and dmise ligand-based oxidation potentials also significantly increase upon Fe(ii) binding compared, indicating that bound thione and selone ligands will undergo oxidation prior to Fe(ii). The synthesis of these complexes suggests that iron coordination by thione and selone ligands may occur in vivo and may contribute to the protective antioxidant properties of sulfur and selenium.